The goal of this project is to investigate the contributions of the left and right cerebral hemispheres towards language processes, in particular, to examine claims regarding the contribution of the right hemisphere to the elaboration of semantic knowledge. The participation of the right hemisphere has long been suspected as a source of semantic paraphasic and paralexic errors, but there has been little theoretical attention to this possibility. We investigate these related positions: 1) lexical semantic procedures (narrowly defined as those aspects of semantic and knowledge that plays a role in syntactic operations) are carried out by left hemisphere processes, but perceptual and conceptual semantic knowledge is shared between hemispheres, 2) the representation of semantic knowledge does not differ between hemispheres, but time course of access to perceptual, conceptual, and lexical processes differ and yields hemispheric effects, and, specifically, 3) automatic spreading activation plays a role in lexical access procedures in both hemispheres, but controlled processing is more characteristic of the left hemisphere. The structure and processing characteristics of left and right hemisphere language will be investigated in three populations; patients who have undergone callosotomy for the control of intractable seizure disorders, patients with focal right and left hemisphere lesions, and normal subjects. Tasks have been designed so that variables such as stimulus presentation, and response required will vary as little as possible to allow comparison of results between studies. Together these experiments should allow us to state more firmly the limits of right hemisphere participation in normal and abnormal language function and to better understand the representation of perceptual, conceptual, and lexical information in both hemispheres. If we are to develop therapeutic strategies that enlist the preserved functions of non-dominant hemisphere in aphasic patients it is crucial that we understand what they are and how they are integrated.